# std::unordered_multimap<Key,T,Hash,KeyEqual,Allocator>::clear

From cppreference.com
< cpp‎ | container‎ | unordered multimap

void clear() noexcept; |  | (since C++11)

Erases all elements from the container. After this call, size() returns zero.

Invalidates any references, pointers, and iterators referring to contained elements. May also invalidate past-the-end iterators.

### Parameters

(none)

### Return value

(none)

### Complexity

Linear in the size of the container, i.e., the number of elements.

### Example

Run this code

    #include <iostream>
    #include <string_view>
    #include <unordered_map>

    void print_info(std::string_view rem, const std::unordered_multimap<int, char>& v)
    {
        std::cout << rem << "{ ";
        for (const auto& [key, value] : v)
            std::cout << '[' << key << "]:" << value << ' ';
        std::cout << "}\n";
        std::cout << "Size=" << v.size() << '\n';
    }

    int main()
    {
        std::unordered_multimap<int, char> container{{1, 'x'}, {2, 'y'}, {3, 'z'}};
        print_info("Before clear: ", container);
        container.clear();
        print_info("After clear: ", container);
    }

Possible output:

    Before clear: { [1]:x [2]:y [3]:z }
    Size=3
    After clear: { }
    Size=0

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 2550 | C++11 | for unordered associative containers, unclear if complexity is linear in the number of elements or buckets | clarified that it's linear in the number of elements

### See also

erase | erases elements (public member function)

Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/container/unordered_multimap/clear&oldid=136064"